I'm not exactly sure why but thats just something to get use to in German writing. It might have something to do with preventing confusion if words mean the same thing or word order? Just always remeber to capitalize your nouns.
There is not exactly a set reasoning behind the gender of nouns, you just need to learn the genders as you learn the words. Example; der Apfle - the apple is masculine. die Zitrone - the lemon is feminine. Learn genders as you go.